Does XTE J1550-564 Exhibit Limit-Cycle Behavior During Outburst Episodes?

X-ray nova (XRN), a black hole binary with transient nature, poses a serious challenge for standard disk theory. Outburst and quiescent phenomena observed from the source cannot be explained by steady-state accretion onto the compact object. Disk instability model (or limit-cycle instability model) was developed mainly to account for the outbursts seen in dwarf-novae, which later was found to work just as well in XRN but with a larger timescale and energy range. XRN XTE J1550-564 provides a better chance for the time-dependent study of the accretion disk around a black hole. This source has undergone multiple outbursts with different intensities and is known to be the only XRN to exhibit "X-ray mini-outbursts". We use RXTE data to analyze X-ray light curves and spectra, especially during its outbursts and mini-outbursts.

The first main outburst has the longest duration (197 days), while the 4 mini-outbursts have an average duration of 42 days. The time spent in the quiescent period for this source was approximately 330 days between outburst episodes. This is much shorter than the typical quiescent timescale (or viscous diffusion timescale) for black hole binaries, indicating that the following mini-outbursts were practically "residual" of the main outburst and not separate ones. Decreasing peak intensity and the absence of disk components in the last three mini-outbursts provide an insight into the physical condition of the accretion disk. The disk has already receded to a large radius and the flux was dominated by a power-law component that might have originated from ADAF or jet.

After those subsequent events of outbursts and mini-outbursts, the source has since gone quiet, likely undergoing a "real" quiescent phase to accumulate material to form an accretion disk. While the values of a few parameters obtained from the data may differ from the theory, the existence of limit-cycle instability in the accretion disk cannot be ruled out. The next outburst of XTE J1550-564 may shed some light on the complexity of this source.